Bedrock and Stream Sediment Geochemical Analysis and Field Observations of the Sub-Cretaceous Unconformity, Northeast Alberta (NTS 74E and North Half 74D)

The objective of this Geo-Note is to make the geochemical results available from 1996, 1997 and 1999 sampling and to report on geological field observations that may be of interest to the minerals industry.

The sub-Cretaceous unconformity in the Fort McMurray area of northeastern Alberta, is defined by the major break in sediment deposition between the lower Upper Devonian Waterways Formation carbonate of the Upper Beaverhill Lake Group and lower Cretaceous McMurray Formation sandstone and siltstone rocks of the lower Mannville Group. The area is thought to be an attractive site for mineralization since a transition from a lower oxidized system (Elk Point Group to Precambrian) to an upper reduced system (upper Beaverhill Lake and lower Mannville groups) exists. The reducing potential of the Beaverhill Lake Group carbonates directly below the sub-Cretaceous unconformity was enhanced by the downward infiltration of highly reducing hydrocarbons from the heavy oil-impregnated lower Mannville Group.

A total of 195 bedrock and core samples of Devonian and Cretaceous rocks were collected during various excursions to the field and to the core facility in Calgary between 1996 and 1999. An additional 64-stream sediment, and 89 stream sediment heavy mineral concentrate (HMC) samples were collected. The majority of these samples were collected to the north of Fort McMurray in the Bitumount 1:250 000 map sheet (NTS 74E) and to the east along the Clearwater River.

A correlation exists between aeromagnetic lineaments, surface fault structures characterized by sideritization and possibly silification, and sulfide mineralization. Disseminated and massive sulfides (pyrite and marcasite) were located 'in situ' and form preferentially along carbon-rich layers and fractures in the carbonate. The formation of these nodules, in addition to marcasite being crystallized from melnikovite, are believed to be indicative of a low temperature sedimentary origin, or the precipitation from low-temperature hydrothermal brine solutions. A green clay layer, which is typically located in karsted sections and separates the Devonian Waterways Formation from the overlying Cretaceous McMurray Formation, contains the highest concentration of metals. These include values of up to 1,342 ppm Zn, 37.4% Fe, 342 ppm V, 105 ppm Cr, 126 ppm Ni, 10 ppb Au and 13 ppb Pd. Ambiguities remain concerning the hydrothermal versus diagenetic origin of both sulfides and the green clay.